Prior to the discovery of the present invention, there have been innumerable aerosol compositions proposed for providing a continuous coating or film for decorative or protective purposes. Aerosol coating compositions using conventional solvent reduced pigmented organic polymeric binders have heretofore found wide acceptance because of their ready availability to apply paint coatings without concomitant clean up problems associated with brushing applications and brush-marks foreign to spray coating applications. Until recently, aerosol paints were substantially and conventionally water-free, solvent thinned products formulated with propellants of the hydrocarbon class.
With the rapid development of water-reducible paint systems characterized by ease of application, ready clean up, and freedom from fire hazard, water-containing systems for aerosol paints are of major interest. Water-containing aerosol spray paints have the advantage of low toxicity levels, non-flammable spray paint systems, and in some cases ready clean up with soap and water after completion of the paint application.
Early formulations of these aerosol water-reducible coating compositions did not, however, contain water as a major component. For example, U.S. Pat. No. 4,187,204, takes advantage of resinous synthetic polymers or paint binders; illustratively, maleinized oils, oil modified alkyds, copolymer oils, epoxy resins, etc., having sufficient carboxyl groups (acid value) to be solubilized in water by neutralization of the acid groups present. The resulting product may contain some water, but not more than 10%. Acid values of the resins were neutralized with ammonia and amines, and dissolved in water-soluble solvents. This reference includes as propellants the lower molecular weight hydrocarbons fluorocarbons, etc., CO.sub.2, nitrous oxide, cyclo-propane, etc. Among the host of non-distinguishable propellants dimethyl ether was named as useful. The useful water content was limited however to preferably less than 8%.
U.S. Pat. No. 4,125,499 uses organic co-solvents including acetone, ethers, ethylene glycol n-butyl ether, etc., with about 5% of water with some water-soluble alkyd resins (Aquamac 1200, Kelsol 3902, etc). Aliphatic hydrocarbons and halofluorocarbons and methyl ethers including dimethyl ether are used in propellant admixtures. Again water is limited to not more than 10%.
Subsequent development of these aqueous-based aerosol coating compositions have yielded increased water content aerosols. For example, U.S. Pat. No. 4,518,734 provides water-based aerosol coatings which comprise water-reducible resins, water in an amount from 10 to 70 wt. percent, a water-soluble solvent and dimethyl ether as a propellant. The resins contemplated by this reference include water-soluble oil-modified alkyds, acrylics, styrene acrylics, urethanes, polyesters, silicone polymers and epoxies in an amount ranging from 5 to 60 wt. percent.
U.S. Pat. No. 4,450,253 discloses a composition of 6 to 25 wt. percent of a water-soluble oil-modified ester polymer, 10 to 40 wt. percent water, 0.1 to 15 wt. percent of a water-soluble polar organic solvent, and 20 to 60 wt. percent of dimethyl ether as a propellant. No other types of resins are contemplated by this reference.
U.S. Pat. No. 4,482,662 provides yet another water-based aerosol composition. This composition includes 5 to 20 wt. percent of a water-soluble monomer modified alkyd resin having an acid value from 30 to 80, 15 to 40 wt. percent water, 5 to 30 wt. percent of a water-soluble organic solvent, and 20 to 45 wt. percent of dimethyl ether. U.S. Pat. No. 4,420,575, which is related to the '662 patent, contemplates an emulsion polymer latex as the film-forming component as well as the water-soluble alkyd resin.
Although the water-reducible resins contemplated in these references are suitable for film-forming, they produce a relatively soft film. Such films are easily damaged when an article onto which the film is applied is used. Further, maintaining the cleanliness of a soft film is more difficult. Various types of foreign materials, e.g., dust and dirt, are more easily embedded into a softer film.
There is an additional limitation on the use of the aforesaid resins. More specifically, there is an upper limit on the weight percent of these resins and other solids in aerosol formulations due to their effect on the viscosity of the aerosol composition. As the amount of resin and other solids is increased, the viscosity increases. As the viscosity of an aerosol composition should be less than about 400 centipoises for acceptable spraying characteristics, and preferably from about 100-200 centipoises, the wt. percent solids of known compositions are typically limited to an amount less than about 30 wt. percent.
Thus, there exists a need for an water-based aerosol coating composition which, after discharge onto a substrate, results in a film which is more easily kept free of embedding contaminants, is more durable than previously existing compositions, and, further, which exhibits favorable characteristics in the area of flammability.